> How can someone possibly report when they are not experiencing consciousness?

By an absence of reporting it. If I sit at a desk getting my neurons moderated by testing equipment and say "I am conscious" every subjective second that I am experiencing consciousness then I could at least help narrow down when consciousness is lost. If I am simply unable to speak or respond at all, but still conscious, I would report that fact later. Only in the case of locked-in conscious awareness without later memory of the experience would this kind of experimental setup fail, and this is where brain imaging could probably help determine that everything except motor or memory neurons were active.